As shown in FIG. 9, in a fluidized-bed incinerator 5, fluidized sand of a fluidized bed 6 constituted by the fluidized sand circularly supplied to the lower portion of the incinerator is preheated to a temperature of about 100.degree. C., and fluidizing air 16 allows the sand to flow. In this state, combustion materials supplied from a charge shoot 2 are combusted for a short time. An unburnt gas on the fluidized bed 6 is combusted by secondary air 15 supplied to a free board portion 13, and a combustion exhaust gas 14 is exhausted from a top portion of the incinerator 5.
Incombustibles contained in the combustion materials are exhausted from the bottom portion of the incinerator together with the fluidized sand. The fluidized sand is separated from the incombustibles and recycled in the incinerator.
In order to control a combustion state of the fluidized-bed incinerator, as is disclosed in Published Unexamined Japanese Patent Application No. 53-14865, the following method is employed. That is, an oxygen concentration in exhaust gas is detected to control a supply rate of combustion materials such as garbage.
A combustion state in an incinerator is determined by brightness or the like in the incinerator. For example, when the combustion becomes strong, a fluidizing air flow rate is decreased to cause the combustion to be weak. This method is disclosed in PCT WO88/8504.